Using in vivo transplantation methods with the aid of histocompatibility markers, cellular composition and cell interactions will be studied in normal and neoplastic tissues derived from experimentally produced genotypically mosaic mouse mammary glands. Three separate populations of cells representing 3 different cell types having separate origin in adult tissues comprise normal breast and give rise to 3 different classes of neoplasia. A reliable system of classification of breast neoplasia based on cellular composition and cell type origin will result from correlation of histopathology with transplantation data. Allogeneic mammary parenchyma grown in stroma syngeneic with transplant recipients grows normally and survives indefinitely. The afferent limb of the immune response is blocked by the intact epithelial-stromal junction. When antigenic tissues are not destroyed by immunity, their prolonged presence results in specific cell-mediated suppression of immunity. Suppression is transferrable by lymphoid cells to recipients syngeneic with the donor of suppression or to recipients allogeneic to both donor and parenchyma when they share major histocompatibility complex products with both. The significance of these phenomena to transplantation and cancer immunity will be studied.